U.S. patent application number 10/121522 was filed with the patent office on 2002-11-07 for electrical machines.
Invention is credited to Lynch, Cedric.
Application Number | 20020163258 10/121522 |
Document ID | / |
Family ID | 10747138 |
Filed Date | 2002-11-07 |
United States Patent
Application |
20020163258 |
Kind Code |
A1 |
Lynch, Cedric |
November 7, 2002 |
Electrical machines
Abstract
An electrical machine, such as an electric motor, dynamo or
alternator has a casing with cooling vents enabling cooling fluid
to flow into and out of the casing when the rotor of the electrical
machine rotates. The rotor may be formed from conductive elements
connected together at their outer regions by inter-connecting
members which have vanes arranged to direct cooling fluid over the
outer regions. Each conductive element is a metal strip with legs
bent in opposite directions relative to the plane of the strip.
Portions of the windings of the rotor are spaced apart to allow
fluid to flow between the windings to enhance the cooling effect.
With a current carrying rotor, varying the axial separation of the
rotor and the stator varies the magnetic field intensity across the
rotor.
Inventors: |
Lynch, Cedric;
(Hertfordshire, GB) |
Correspondence
Address: |
Alfred Basichas, Esq.
Morgan, Lewis & Bockius LLP
101 Park Avenue
New York
NY
10178
US
|
Family ID: |
10747138 |
Appl. No.: |
10/121522 |
Filed: |
April 12, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10121522 |
Apr 12, 2002 |
|
|
|
09490829 |
Jan 25, 2000 |
|
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Current U.S.
Class: |
310/58 |
Current CPC
Class: |
H02K 5/20 20130101; H02K
13/08 20130101; H02K 5/207 20210101; H02K 23/44 20130101; H02K 3/04
20130101; H02K 9/06 20130101; H02K 3/24 20130101; H02K 23/54
20130101 |
Class at
Publication: |
310/58 |
International
Class: |
H02K 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 1993 |
GB |
9326353 |
Claims
1. A conductive rotor for an electrical machine, having a
current-carrying winding comprising a plurality of
circumferentially distributed winding portions which lie in at
least one winding plane perpendicular to the rotor axis, and extend
from a radially inner region to a radially outer region, and a
commutator provided by surfaces of the winding portions, the
winding being formed of a plurality of integrally formed conductive
elements each comprising a metal strip, in which the surfaces
providing the commutator are edge surfaces of the metal strip, said
edge surfaces being spaced apart from each other to allow fluid
flow therebetween for cooling the commutator.
2. A conductive rotor as claimed in claim 1, in which the major
surfaces of the metal strip are coated adjacent the commutator with
an insulating material.
3. A conductive rotor as claimed in claim 2, wherein said
insulating material has a relatively low wear resistance so that it
will readily wear or break away from said major surfaces at their
portions adjoining said edge surfaces as the commutator wears
during use.
4. A conductive rotor as claimed in claim 3, wherein said
insulating material is a coating formed by baking a powder coating
applied to said conductive sections.
Description
[0001] This is a Continuation of U.S. patent application Ser. No.
09/490,829, filed Jan. 25, 2000, and the contents of which are
incorporated by reference herein, as if restated in full.
BACKGROUND
[0002] The present invention relates to electrical machines, which
convert mechanical energy into electrical energy, or vice versa, by
an interaction between a magnetic field and an electric current. In
particular, the present invention relates to a casing for such an
electric machine, a rotor for such an electric machine, a
conductive element of a rotor winding for such an electric machine,
and a method of forming such a conductive element. Examples of such
electric machines are electric motors, dynamos and alternators.
[0003] In known electric machines, the assembly of a rotor winding
may be time consuming and therefore expensive. Moreover the rotor
winding often comprises a large number of different parts, which
increases the overall cost of the electric machine.
[0004] The performance of such electric machines may also be
limited by the amount of heat generated in the rotor winding and in
the region of the commutator. As a result of this limited
performance, the range of applications of such electric machines,
particularly in electrically powered vehicles, has been
restricted.
SUMMARY OF THE INVENTION
[0005] According to one aspect of the present invention, there is
provided a conductive element of an armature for an electrical
machine, comprising a metal strip having a pair of leg portions
joined together at or about one end by a flat bridging portion, the
first and second leg portions being bent in opposite directions
perpendicularly to the plane of the bridging portion.
[0006] The armature may be easily constructed by arranging a number
of such conductive elements with even circumferential spacing
around a circle, to form the structure of the armature.
[0007] Preferably the first and second leg portions have a
substantially equal width so that the assembled rotor consists of
two winding planes of equal thickness, perpendicular to the axis of
rotation.
[0008] However, in an alternative embodiment each conductive
element may have three leg portions joined together at or about one
end to a bridging portion, the two outer leg portions on either
side of the bridging portion being bent in the same direction
perpendicularly to the plane of bridging portion, and the middle
leg portion being bent in an opposite direction.
[0009] In this alternative embodiment, the middle leg portion
preferably has twice the width of each of the outer leg portions,
so that the armature, when assembled, has three winding planes
perpendicular to the axis of rotation, the middle winding plane
having twice the thickness of each of the outer winding planes.
This construction reduces shearing between the winding planes at
high rotation speed and therefore reduces the risk of damage to the
armature.
[0010] Preferably, each of the leg portions includes a radial
portion, in which the current carried in the winding interacts with
an applied magnetic field, and an outer portion which is bent
towards the tangential direction of the armature so that it may be
joined to another conductive element displaced around the
circumference of the armature.
[0011] Preferably, the above-mentioned conductive element is
stamped from metal sheet and the first and second leg portions are
bent in opposite directions perpendicular to the metal sheet. In
the method of forming the conductive element having three leg
portions, the middle leg portion is bent in one direction
perpendicular to the metal sheet, while the outer leg portions are
bent in an opposite direction.
[0012] According to another aspect of the present invention, there
is provided a conductive armature for an electrical machine, having
a current-carrying winding formed from a plurality of integrally
formed conductive elements circumferentially distributed around the
armature, in which the radially outer portions of adjacent
conductive elements have a gap between them to allow cooling fluid
to flow through the radially outer portion of the armature. Each
conductive element has a radially outer portion bent towards the
tangential direction of the armature; the gap between adjacent
conductive elements extending along a substantial portion of the
length of the radially outer portions.
[0013] Adjacent ones of the conductive portions abut against each
other in a radially inner area of the radially outer portions.
Thus, cooling fluid is contained within the radially outer portion
of the conductive elements, thereby enhancing cooling in the
radially outer portions.
[0014] According to another aspect of the present invention, there
is provided a conductive armature for an electrical machine, having
a current-carrying winding comprising a plurality of
circumferentially distributed integrally formed conductive
elements, in which the surfaces providing the commutator are edge
surfaces of the integrally formed conductive elements, adjacent
ones of the conductive elements being spaced apart in the
commutator area to allow cooling fluid to flow between the
conductive elements in that area. As a result, greater cooling can
be achieved in the commutator area, and brush dust, insulator and
other debris is removed by the flow of cooling fluid.
[0015] Preferably, the major surfaces of the conductive elements in
the commutator area are coated with an insulating material, which
is brittle or has relatively low wear resistance. As a result,
electrical contact between adjacent conductive elements in the
commutator area, caused for example by conductive brush dust, is
prevented, while the insulating coating is worn down by contact
with brushes which contact the commutator, in order to maintain a
good contact between the brushes and the commutator. The insulating
material, which is worn away, together with brush dust and other
debris, may then be removed by the cooling fluid flowing through
the spaces between the conductive elements in the commutator
area.
[0016] According to a further aspect of the present invention,
there is provided a casing for an electrical machine having a
rotor, the casing having cooling apertures to allow cooling fluid
to flow into the casing, through the rotor and out of the casing
when the rotor rotates, at least some of the apertures being
louvers located in the radially outer portion of the casing, the
louvers being inclined to direct out of the casing cooling fluid
which circulates in the casing when the rotor rotates. Others of
the apertures may be also located in a radially outer portion of
the casing and may be louvers inclined to direct cooling fluid into
the casing when the rotor rotates. In this way, the flow of cooling
fluid is driven through the casing by the action of the rotor, so
that the rotor is self-cooled. Moreover, the cooling fluid flow is
driven when the rotor rotates in either direction.
[0017] Preferably, the louvers inclined in a first sense are
arranged in a first plane perpendicular to the axis of rotation of
the rotor, while the louvers inclined in a second, opposite sense
are arranged in a second plane parallel to the first plane, so that
cooling fluid is directed through the casing and through the rotor
with an axial component when the rotor rotates.
[0018] Alternatively, apertures may be disposed in a radially inner
portion of the casing, whilst louvers are disposed in a radially
outer portion of the casing, arranged to direct air out of the
casing when the rotor rotates. Thus, air is drawn into the
apertures and directed out of the louvers, the flow of cooling
fluid from the apertures to the louvers being assisted by the
centrifugal force on the fluid circulating within the casing.
[0019] Preferably, the casing may include a fluid passage having an
inlet arranged on the opposite side of the rotor from the
apertures, and an outlet arranged to direct cooling fluid onto a
radially outer portion of the rotor. Thus, the cooling fluid flows
through the rotor from the apertures to the inlet of the fluid
passage and is directed in an axial direction at a radially outer
portion of the rotor towards the louvers. In this embodiment, the
action of the louvers reduces the pressure of cooling fluid within
the casing, thereby drawing cooling fluid into the apertures at the
radially inner portion. Because the louvers are inclined in one
sense only, this embodiment is only effective in one direction of
rotation of the rotor.
[0020] Preferably, the rotor for use with the casing has gaps at
its radially inner and radially outer portions, which permit
cooling fluid to flow axially through the rotor at these portions.
Preferably, the rotor is a disc rotor with either the radially
inner or radially outer portions of either or both of the faces
providing the commutator, so that the cooling fluid flows through
the rotor in the commutator area.
[0021] According to another aspect of the present invention, there
is provided a rotor for an electrical machine, having a
current-carrying winding comprising a plurality of
circumferentially distributed conductive elements each having ends
which lie at a radially outer region of the rotor, interconnections
between the ends being made by interconnecting members having vanes
arranged to direct cooling fluid axially across the ends.
[0022] According to another aspect of the present invention, there
is provided an electric machine comprising a current-carrying rotor
and a stator for producing an axial magnetic field through the
rotor, the axial spacing between the rotor and the stator being
variable, so as to vary the magnetic field in the rotor.
[0023] Preferably the stator comprises first and second sets of
permanent magnets arranged on opposite sides axially of the rotor,
the first and second sets of permanent magnets being supported
respectively on first and second support members coupled to move in
opposite axial directions relative to the rotor.
[0024] The separation between the first and second support members
may be varied by camming means arranged between the members, or by
one or more screw threaded rods engaging the first and second
support members.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] Specific embodiments of the present invention will now be
described with reference to the accompanying drawings, in
which:
[0026] FIG. 1 is a cross-section in an axial plane of an electric
motor embodying aspects of the present invention;
[0027] FIG. 2a is a schematic view in the axial direction of the
rotor of the electric motor shown in FIG. 1;
[0028] FIG. 2b shows a detail of the rotor of FIG. 2a;
[0029] FIG. 3a is a view in an axial direction of a single
conductive element;
[0030] FIG. 3b is a view of the conductive element of FIG. 3a in a
radial direction;
[0031] FIG. 3c is a plan view of a blank from which the conductive
element of FIGS. 3a and 3b is formed;
[0032] FIG. 4 is a plan view of a blank from which an alternative
conductive element is formed;
[0033] FIGS. 5a to 5e are plan views of alternative conductive
elements showing commutating points;
[0034] FIG. 6a is a side elevational view in an axial direction of
a casing in accordance with the present invention;
[0035] FIG. 6b is a view in a radial direction of the casing of
FIG. 6a;
[0036] FIG. 7 is a cross-sectional view through the axis of
rotation of art electrical machine having another embodiment of a
casing in accordance with the present invention;
[0037] FIG. 8 is a view in a radial direction of a portion of a
rotor in accordance with one aspect of the present invention;
and
[0038] FIG. 9 is a diagrammatic end view of an electrical machine
in accordance with a further aspect of the present invention.
DETAILED DESCRIPTION
[0039] FIG. 1 shows an electric motor having a rotor 10, through
which current flows, and two sets of permanent magnets 12 and 13
arranged on respective opposite axial sides of the rotor 10. The
magnets 12 and 13 are mounted on respective first and second stator
plates 14 and 16. Each of the stator plates thus carries a set of
permanent magnets arranged in a circle with alternately opposite
magnetic poles thereof facing the rotor. The circles of magnets are
arranged in mutual register so that each magnet 12 on one side of
the rotor is aligned in a direction parallel to the rotor axis with
a corresponding magnet 13 on the other side of the rotor. Each such
pair of opposite magnets 12, 13 is arranged with opposite magnetic
poles facing the rotor, as shown in FIG. 1. The magnets may be
ferrite magnets or rare earth element magnets.
[0040] Electric current is supplied to the rotor 10 through brushes
(not shown) at commutator points P.sub.1.
[0041] The rotor 10 includes a dove-tailed portion 18 which is
fitted onto a hub 20 which is shaped with an annular inclined
shoulder 20' to inter-engage with the dove-tailed portion 18. The
rotor 10 is clamped onto the hub 20 by a cap 22, which is bolted
onto the hub 20. The hub 20 is connected to a spindle 17, which is
rotatably mounted in the first stator plate 14 by means of bearings
24. The first and second stator plates 14, 16 are joined by spacer
rods 25 and the spacing between them is closed by a cylindrical
casing 26.
[0042] Referring also to FIGS. 2a and 2b, the rotor 10 is
constructed as a disc from a number of circumferentially spaced
winding portions. First winding portions 28, shown in solid
outline, are arranged in a first plane W.sub.1 perpendicular to the
axis of the rotor, while second winding portions 30, shown in
dotted outline, are arranged in a second plane W.sub.2 parallel to
the first plane, behind the first winding portions 28. Each winding
portion 28, 30 includes a radially extending section 32, through
which the magnetic field passes, a radially inner section 34 bent
in one circumferential sense at a shallow angle to the radially
extending section 32, and a radially outer section 36 bent in the
opposite circumferential sense at a substantial angle .theta. less
than 90.degree. relative to the radially extending section 32
towards the tangential direction. The radially outer section 36
terminates in an outwardly turned end section 38. The end section
38 of each first winding portion 28 in the first plane W.sub.1 is
connected to an adjacent end section 38 of a second winding portion
30 in the second plane W.sub.2 and at the same circumferential
position by a connecting cap 40 soldered onto the end sections 38.
As the first and second winding portions are bent in opposite
directions, the radially extending section 32 of the second winding
portion 30 is displaced around the circumference of the rotor from
the radially extending section 32 of the first winding portion 28
by an angle approximately equal to the pitch of the permanent
magnets 12 or 13 on the respective stator plates 14 or 16. Thus,
the electromagnetically generated tangential forces on the radially
extending sections 32 of the first and second winding portions 28
and 30 are in the same sense. Each winding portion 28 is also in
electrical contact with a second winding portion 30 at their
radially inner sections 34 so that the current path alternates
between the first and second winding portions 28 and 30.
[0043] The current path around the rotor is shown by arrows in FIG.
2a, which shows that starting at an arbitrary starting point, such
as point P.sub.8 at the inner end of one first winding portion 28,
the current path in one cycle around the rotor 10 does not return
to that starting point but to an adjacent point P.sub.9 displaced
by one winding pitch from the starting point P.sub.8. Thus, the
first and second winding portions 28, 30 are connected together in
a continuous current-carrying loop to form the wave-wound rotor
10.
[0044] The magnetic field strength across the radially extending
sections 32 is enhanced by laminations 42 of a material having high
magnetic permeability, for example mild steel, silicon steel or
soft iron, which are inserted in stacks in the spaces between
radially extending sections 32 of the winding portion 28 and 30. As
shown in greater detail in FIG. 2b, an insulating wrapper 43 is
first placed in the spaces and then the stacks are placed within
it.
[0045] However, gaps G.sub.1 between adjacent radially outer
sections 36 and gaps G.sub.2 between adjacent radially inner
sections 34 are kept open to allow air, constituting a cooling
fluid, to flow between the winding portions 28, 30.
[0046] Fluid flow through the gaps G.sub.1 may be guided by
allowing the radially outer sections 36 of adjacent winding
portions 28, 30 to contact each other at a section b, which in this
case is at the junction bend between the radially extending section
32 and the radially outer section 36. Thus, cooling fluid flows
axially between the radially outer sections 36, where a large
surface area of the winding portions 28, 30 is exposed and is
prevented from flowing past the section b in a tangential
direction.
[0047] In one example, the section b extends over approximately a
third of the length of the radially outer sections 36, with the
remaining two-thirds of the length being open to the cooling
fluid.
[0048] Furthermore, to ensure a strong mechanical bond between the
winding portions, the sections b between each of the winding
portions 28, 30 are impregnated with resin 41. Thus, a continuous
bonded section B is formed extending circumferentially completely
around the rotor 10, as shown in FIG. 2a.
[0049] As shown in FIGS. 3a to 3c, a pair of first and second
winding portions 28, 30 are formed integrally from a. conductive
element 44. Each conductive element 44 is formed from a blank 45 of
metal strip which is stamped from a metal sheet and comprises a
pair of parallel strip portions in the form of legs 46 and 48 which
form respectively the first and second winding portions 28 and 30.
The leg portions 46 and 48 are joined together at their radially
inner ends by a bridging portion 50. The dove-tail section 18 of
the rotor 10 is formed in the bridging portion 50 by stamping
indentations 18' in either side. The legs 46 and 48 are bent in
opposite directions D.sub.1, D.sub.2 perpendicular to the plane of
the flat bridging portion 50 and are suitably bent to form the
first and second winding portions 28, 30 having radially inner
sections 34, radially extending sections 32, radially outer
sections 36 and end sections 38.
[0050] Each conductive element 44 is powder coated with insulating
material, such as epoxy resin, in all areas apart from the end
sections 38 and baked to form an insulating coating on the element
before assembly.
[0051] To assemble the rotor, the required number (129 in a
particular example) of such bent conductive elements 44 are
mutually positioned in a nesting arrangement with
circuinferentially equal spacing to form the disc structure of the
rotor and the connecting caps 40 are soldered onto the end sections
38. Then the rotor 10 is placed on the hub, with the dove-tail
portion 18 resting against the inter-engaging shoulder portion 20'.
The cap 22 is fitted onto the hub 20 so that the dove-tail portion
18 is clamped-between the cap 22 and the inter-engaging 20 shoulder
portion 20'. The laminated pieces 42 are then inserted in the ring
of gaps between the radially extending sections 32.
[0052] In a particularly advantageous arrangement, edge portions of
the conductive elements form the commutator of the electric motor,
these being edge portions of either the radially inner sections 34
or the radically outer sections 36. The insulating coating on the
edges of the conductive elements 44 is removed from the commutator
area before use. Alternatively, the edge portions may be masked
during the powder coating operation so that the insulating powder
is not deposited on these portions. The insulating material
remaining on the broad faces of the conductive elements 44 is
brittle or has a low resistance to wear so that, as the edges of
the elements 44 are worn by contact with the brushes, the
insulating material on the faces is also worn away at the face
portions adjoining these edge portions forming the commutator and
does not prevent the brushes from contacting the commutation. As
the insulating material is worn away, it disintegrates into
particles and is carried away by the cooling fluid flowing through
the gaps G.sub.1 or G.sub.2, and therefore does not accumulate in
the commutator area.
[0053] FIG. 4 shows an alternative form of blank 45' having three
legs 51, 47 and 49. The width of the middle leg 47 is twice that of
each of the outer legs 51 and 49. The outer legs 51 and 49 are bent
in the same direction perpendicular to the strip, while the middle
leg 47 is bent in an opposite direction to form an alternative
conductive element. When assembled, the alternative conductive
elements form a rotor in which the legs 51 and 47 form first and
second winding portions 28 and 30 in first and second winding
planes W.sub.1, and W.sub.2, while the leg 49 forms a third winding
portion in a third winding plane parallel to the first and second
winding planes.
[0054] The three-plane construction reduces the problem of shearing
between winding planes at high speeds, which is caused by the
winding portions tending to straighten under high centrifugal
forces. The division of the rotor into more winding planes of
narrower width reduces the shearing forces between adjacent
planes.
[0055] FIG. 5a shows several possible points P.sub.1, P.sub.2,
P.sub.3 on the edges of each conductive element 44 which may form
the commutator area. Points P.sub.1 are on either outer edge of the
radially inner section 34, points P.sub.2 are on either outer edge
of the radially outer section 36, and point P.sub.3 is on the
radially outer edge of the end section 38.
[0056] FIG. 5b shows a modified metal strip in which one side of
the radially outer section 36 is chamfered to form a commutator
edge at an angle relative to the face of the rotor 10. The brushes
contact the commutator edge at the point P.sub.4.
[0057] The position of the commutator may be varied further by
stamping the blank 45 in a form, which includes a lateral
projection from one side. In the form shown in FIG. 5c, the lateral
projection is located at the radially outer section 36, and in the
assembled rotor will form a peripheral, axially projecting ring
commutator which will, be contacted by the brushes at the point
P.sub.5, while in FIG. 5d, the lateral projection is at the
radially inner section 34 and will form an inner, axially
projecting ring commutator adjacent the hub for contact by the
brushes at the point P.sub.6. In another possible form shown in
FIG. 5e, the radially inner lateral projection is chamfered to form
an angled commutator edge at the point P.sub.7. The various
possible commutator points P.sub.1, P.sub.2, P.sub.3, P.sub.4,
P.sub.5, P.sub.6, P.sub.7 provide flexibility in the location of
the brushes and therefore in the design of the electrical
machine.
[0058] Referring to FIGS. 6a and 6b, the cylindrical casing 26
which is attached to the outer edges of the stator plates 14, 16 is
provided with a first set 52 and a second set 54 of louvers
inclined in opposite directions relative to the tangential
direction of the rotor. Each set comprises a ring of
circumferentially spaced louvers, which are formed by cutting or
punching the cylindrical casing 26 to form apertures and surfaces
inclined in one or the other tangential direction. When the rotor
rotates in the direction R, air is drawn into the casing 27 through
the first set of louvers 52, and is expelled from the casing
through the second set of louvers 54. On the other hand, when the
rotor rotates in the opposite direction R', air is drawn into the
casing through the second set of louvers 54 and expelled through
the first set of louvers 52.
[0059] As shown in FIG. 6b, the rings of the first and second sets
of louvers 52 and 54 are spaced apart from each other in the axial
direction. Thus, the airflow F through the casing has an axial
component and is forced through the gaps G.sub.1 of the rotor
10.
[0060] In an alternative cooling arrangement shown in FIG. 7, the
first stator plate 14 has a set of apertures 56 formed in a ring at
a radially inner position. Air flows through the apertures 56 and
the gaps G.sub.2 at the radially inner part of the rotor into an
annular air passage 58 or space through inlets 60 formed at a
radially inner part of the second stator plate 16. An outer cover
plate 61 closes the passage or space 58. The air flows radially
through the air passage 58 and thence back into the space between
the stator plates through outlets 62 formed at a radially outer
part of the second stator plate 16. The air then passes through the
gaps G.sub.1 and out of the casing through louvers 64 which are all
inclined to force air out of the casing 26. The air passage 58 is
closed to the surroundings of the casing 26 and therefore the
action of the louvers 64 on the air rotating with the rotor just
inside the casing is to create a suction which draws air into the
apertures 56 and through the air passage 58. In this embodiment,
brushes 66 contact the edges of the winding portions 28, 30 in the
area of the gaps G.sub.2, so that air flows through the commutator
area. Moreover, air flows through the rotor in the commutator area
in a direction towards the commutator surface and flows over the
brushes in a direction away from the rotor, thereby carrying brush
dust away from the commutator area.
[0061] As shown in FIG. 8, two adjacent peripheral interconnections
between end sections 38, viewed end-on in an inward radial
direction, in an alternative embodiment the connecting caps 40
project beyond the rotor in the axial direction and the projecting
portions are angled to form a first vane 68 inclined towards the
direction of rotation R and a second vane 70 inclined away from the
direction of rotation R. Thus, the vanes 68 and 70 act as fan
blades to direct the flow F of air through the gap G.sub.1, thus
enhancing the cooling effect.
[0062] The combination of the rotor shown in FIGS. 2a and 2b with
the casing shown in either FIGS. 6b or FIG. 7 and optionally the
connecting caps 40 shown in FIG. 8, provides a particularly
advantageous cooling effect in which cooling fluid is directed
through sections of the rotor in which the cooling effect of the
fluid is optimized.
[0063] To allow mechanical variation of the running speed of the
electric motor, the spacing between the magnets 12, 13 and the
rotor 10 may be adjusted as shown in FIG. 8 so as to vary the
strength of the magnetic field applied to the rotor 10. In this
embodiment, the magnets 12, 13 are mounted on first and second
axially movable stator plates 74 and 76. The axially movable stator
plates 74, 76 are biased towards each other by the mutual magnetic
attraction of the magnets 12 and 13 which are arranged with
opposite poles facing each other. The plates 74, 76 can be forced
apart by rotatable cams 72 which contact the plates 74, 76.
Alternatively, the separation between the movable plates 74 and 76
may be adjusted by one or more screw threaded rods which pass
through apertures in the plates 74, 76, and have opposite handed
screw threads in the sections which engage the first and second
stator plates 74, 76. As a further alternative, the separation may
be adjusted by one or more screw-threaded rods which have a single
screw thread which engages one of the stator plates 74, 76 and
which abuts the other of the stator plates 74, 76. The stator
plates 74, 76 are held in position by springs or by the mutual
magnetic attraction of the magnets 12 and 13.
[0064] While the above embodiments include a current-carrying rotor
and a magnet-carrying stator, it is clear that the current-carrying
member could be held stationary, and the magnetic portion allowed
to rotate. Although the stator described above carries permanent
magnets, electromagnets may also be used.
[0065] Thus, the present invention provides a rotor for an electric
machine, which is inexpensive to manufacture, and a casing for a
self-cooling electrical machine that does not require separate
cooling means to achieve high performance.
* * * * *